The previously known methods for removing or reducing the concentration of dilute contaminants in the atmosphere are reviewed in U.S. Pat. No. 4,025,605 issued to the present inventors. The disadvantages encountered in the therein listed prior art methods are avoided by the process advocated in the said U.S. Pat. No. 4,025,605. As therein described, the oxidizable organic contaminants are removed from an oxygen-containing inert gas by adsorption in a reaction zone on a sorptive medium, such as adsorbent carbon, in the presence of an oxidation catalyst. The rich adsorbent is then thermally desorbed and oxidized to innocuous reaction products which can be safely discharged to the atmosphere.
The oxidation catalyst employed in the reaction zone described in said patent is platinum or palladium or a salt of either of these, enabling oxidative destruction of the contaminants at lower temperatures than heretofore required and without significant combustion of the carbon adsorbent. The temperature range set out in the patent is from as low as 90.degree. C. to an upper limit prescribed by the ignition temperature of the particular carbonaceous adsorbent. Temperatures in the range of 90.degree. C. to about 200.degree. C. can be employed and preferably in the range of 120.degree.-180.degree. C. Under these conditions for oxidation of the desorbed contaminants the adsorbent capacity of the adsorbent is regenerated.
Among the useful applications of the invention described in the aforesaid patent is that of removing residual vinyl chloride in exhaust streams from polyvinyl chloride synthesis plants.
The several examples of the patent relate to the use of PdCl.sub.2 on activated carbon for the adsorption of the VCM (vinyl chloride monomer) from an air stream, and for subsequent catalytic oxidation of the desorbed VCM. In the case of palladium chloride as catalyst in the reaction of the vinyl chloride monomer, hydrolysis may occur in addition to oxidation. Accordingly, it is desirable, in the case of palladium chloride catalysts, to have water vapor present in the reaction zone during the oxidation reaction. This water vapor may come from the gaseous feed or may be added at some point prior to or during the reaction step in the cycle.
While the method described in the aforesaid patent has been found effective in many of its applications, certain difficulties have been encountered when the contaminant in the air or other oxygen-containing inert gas stream is a vinyl halide, such as vinyl chloride. In such case, the palladium halide catalyst becomes progressively deactivated at a relatively rapid rate.